Package downloader API change

C:

* lr_packagetarget_new() got new param ``LrHandle handle`` (could be NULL)
* ``LrHandle handle`` param was removed from the lr_download_packages()

Python:

* PackageTarget class constructor has a new param ``handle``

* Handle.download_packages(targets, failfast=False) method removed
* Instead use librepo.download_packages(targets, failfast=False) function
* Handle sould be passed to each PackageTarget during construction
* Handle (could be omitted)

Note:
When handle is NULL (omitted), then complete url must be specified
or base_url used.
Add test for downloading multiple packages from multiple repositories